This project is aimed at developing a unified theory of how children learn the vocabulary of their first language and learn to ascertain the meanings of sentences as they are perceived. At its most general level, the proposed research program explores how these "two" learning procedures - the one for the meanings of words, the other for how these meanings combine to yield the meanings of sentences - interact and mutually support each other in a bootstrapping process whose outcome is the mature language user. Much of the work proposed under this grant uses a relatively new experimental technique with children, developed in the Pl's lab under the initial funding period of this grant, in which children's eye gaze is tracked as they hear spoken descriptions of the surrounding visual world. Specifically, children hear instructions that require them to make an implicit choice about the structural organization of the utterance (a 'syntactic ambiguity'). By manipulating potentially informative cues to the intended structure (e.g., verb information, prosodic (tune) information and situational/discourse cues), children's eyegaze and other behaviors can reveal their sensitivity to and representation of these sources. In the initial funding period, our work strongly supported a (probabilistic) constraint-satisfaction lexicalist theory, of both learning and language use. By "lexicalist" we mean a picture of the architecture of language knowledge in which structural and semantic representations exploited in comprehension reside primarily in the meaning of each word. In the upcoming funding period, we propose: (a) to expand and test our developmental account of how children learn to recover the grammatical properties of a sentence as it is heard, by examining eye gaze responses to ambiguous sentences at different ages; (b) to explore how multiple linguistic and non-linguistic cues regarding speaker's intentions are used by the child to uncover word and sentence meaning; (c) to examine what is tracked by the child regarding the meaning of verbs and other relational lexical items as they hear them. An understanding of children's language processing abilities has long eluded researchers, primarily because of technical limitations on tests suitable to use with children. The eyetracking technology used here provides a new window into on-going language interpretation processes, supplying insights into normal development that have potential applications to the treatment of language-development delays and pathologies.